Page 58 - Issue 2_2018
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Most breeds start from a small founding population, and consequent- If two parents are both heterozygous (both Aa) for a gene pair, on the
ly have a high average inbreeding coefficient. If the breed is healthy average, they would produce 25% AA, 50% Aa, and 25% aa. (These
and prolific, the breadth of the gene pool increases, and the average are averages when many litters are combined. In reality, any variety
inbreeding coefficient can go down over time. Some dog breeds of pairing up can occur in a single litter.) If a prolific male comes out
were established on a working phenotype, and not on appearance. of this litter, and he is homozygous aa, then the frequency of the “a”
These breeds usually start with low inbreeding coefficients due to the gene will increase in the population, and the frequency of the “A” gene
dissimilar backgrounds of the founders. As certain individuals are line- will decrease. This is known as the popular sire syndrome. Of course,
bred on to create a uniform physical phenotype, the average inbreed- each individual has thousands of genes that vary in the breed, and
ing coefficient can increase. everyone carries some deleterious recessive genes. The overuse of
individual breeding animals contributes the most to decreased diver-
There is no specific level or percentage of inbreeding that causes im- sity (population bottlenecks), and the increased spread of deleterious
paired health or vigor. If there is no diversity (non-variable gene pairs recessive genes (the founders effect). Again, it is selection (use of this
for a breed) but the homozygote is not detrimental, there is no effect stud to the exception of others), and not the types of matings he is in-
on breed health. The characteristics that make a breed reproduce true volved in that alters gene frequencies. Breeders should select the best
to its standard are based on non-variable gene pairs. There are pure- individuals from all lines, so as to not create new genetic bottlenecks.
bred populations where smaller litter sizes, shorter life expectancies,
increased immune-mediated disease, and breed-related genetic dis-
ease are plaguing the population. In these instances, prolific ancestors
have passed on detrimental recessive genes that have increased in fre-
quency and homozygosity. With this type of documented inbreeding
depression, it is possible that an outbreeding scheme could stabilize
the population. However, it is also probable that the breed will not
thrive without an influx of new genes; either from a distantly related
(imported) population, a natural landrace population, or crossbreed-
ing.
Fortunately, most breeds do not find themselves in the position of this
amount of limited diversity and inbreeding depression. However, the
perceived problem of a limited gene pool has caused some breeders
to advocate outbreeding of all individuals. Studies in genetic conser-
vation and rare breeds have shown that this practice actually contrib-
utes to the loss of genetic diversity. By uniformly crossing all “lines” in a
breed, you eliminate the differences between them, and therefore the Decisions to linebreed, inbreed or outbreed should be made based
diversity between individuals. Eventually, there will not be any “unre- on the knowledge of an individual’s traits and those of its ancestors.
lated line” to be found. Everyone will have a mixture of everyone else’s Inbreeding will quickly identify the good and bad recessive genes the
genes. This practice in livestock breeding has significantly reduced parents share, based on their expression in the offspring. Unless you
diversity, and caused the loss of unique rare breeds. have prior knowledge of what the offspring of milder linebreedings
on the common ancestors were like, you may be exposing your litters
A fallacy of using outbreeding to maintain genetic diversity is the (and buyers) to extraordinary risk of genetic defects. In your matings,
belief that the diversity of a breed must be maintained in every single the inbreeding coefficient should only increase because you are spe-
animal. Breeders must concentrate on the specific goals of breeding cifically linebreeding (increasing the percentage of blood) to selected
(selecting for the health and quality of the breed), versus the tools ancestors.
used to get there (outbreeding, linebreeding, etc.) Selecting breeding
stock simply to produce the lowest possible inbreeding coefficient is Don’t set too many goals in each generation, or your selective
not a goal that will guarantee a quality animal. Animals who are poor pressure for each goal will necessarily become weaker. Genetically
examples of the breed should not be used simply to maintain diver- complex or dominant traits should be addressed early in a long-range
sity. Related individuals with desirable qualities will maintain diversity, breeding plan, as they may take several generations to fix. Traits with
and improve the breed. major dominant genes become fixed more slowly, as the heterozy-
gous (Aa) individuals in a breed will not be readily differentiated from
The process of maintaining healthy “lines” or families of animals, with the homozygous-dominant (AA) individuals. Desirable recessive traits
many breeders crossing between lines (outbreeding) and breeding can be fixed in one generation because individuals that show such
back (linebreeding) as they see fit maintains diversity in the gene pool. characteristics are homozygous for the recessive genes. Individuals
It is the varied opinion of breeders as to what constitutes the ideal that pass on desirable traits for numerous matings and generations
representative of the breed, and their selection of breeding stock should be preferentially selected for breeding stock. This prepotency
that maintains breed diversity. is due to homozygosity of dominant (AA) and recessive (aa) genes.
However, these individuals should not be overused, to avoid the
A basic tenet of population genetics is that gene frequencies do not popular sire syndrome.
change from the parental generation to the offspring. This will occur
regardless of the homozygosity or heterozygosity of the parents, or Breeders should plan their matings based on selecting toward a breed
whether the mating is an outbreeding, linebreeding, or inbreeding. standard, based on the ideal temperament, performance, and confor-
This is the nature of genetic recombination. Selection, and not the mation, and should select against the significant breed related health
types of matings used affect gene frequencies and breed genetic issues. Using progeny and sib-based information to select for desirable
diversity. traits, and against detrimental traits will allow greater control.
Reprinted with Permission 2018
Jerold S Bell DVM
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